Conventional printers can typically be classified as either electrophotographic printers or ink jet printers. Ink jet printers, for example, typically include recording heads, referred to hereinafter as printheads, which employ transducers that utilize kinetic energy to eject ink droplets. A thermal printhead, for example, rapidly heats thin film resistors (or heaters) to boil ink, thereby ejecting an ink droplet onto a print receiving medium, such as paper. According to this ink jet method, upon firing a resistor, a current is passed through the resistor to rapidly generate heat. The heat generated by the resistor rapidly boils or nucleates a layer of ink in contact with or in proximity to a surface of the resistor.
The nucleation causes a rapid vaporization of the ink vehicle, creating a vapor bubble in the layer of ink. The expanding vapor bubble pushes a portion of the remaining ink through an aperture or orifice in a plate, so as to deposit one or more drops of the ink on a print receiving medium, such as a sheet of paper. According to one embodiment of an ink jet printer, by moving the printhead relative to the print receiving medium, a swath of ink drops can be provided on the print receiving medium to form an image, or part of an image, thereon.
Typically, in the aforementioned embodiment, the print receiving medium is kept stationary while the printhead traverses and deposits ink drops. In such an embodiment, when the printhead reverses directions, the print receiving medium is indexed (e.g., advanced forward) in preparation for the next printhead traverse. Thus, a plurality of print swaths can be utilized to create images that are larger in dimension than a single print swath.
As can be understood, properly positioning the print receiving medium relative to the printhead for each print swath can be a critical factor. For example, indexing the medium too far might result in a gap or “white” band (assuming the print receiving is white) between print swaths, while indexing the medium too little might result in overlapping swaths that could create dark horizontal bands in the resulting image. When printing graphic objects or photographic-like images, where even very subtle hue shifts can be detectable by an observer, such swath misplacements can be particularly noticeable, and therefore negatively affect the resultant print quality.
Swath misplacements can be attributed to a variety of factors, such as indexing errors, for example. In particular, indexing errors are believed to occur as a result of the cumulative effect of tolerances associated with parts in the indexing system, which tend to prevent a perfect linear relationship between operation of the indexing system (e.g., rotation of the feed roller) and resultant indexing of the print receiving medium. For example, in one embodiment of a media indexing system including a feed roller and a drive motor, motor positioning errors, gear eccentricities and tooth-to-tooth errors, bearing clearances, media slippage in the feed roller nip, and the eccentric mounting and diameter variation of the feed roller can contribute to indexing errors. Although one approach to reducing indexing errors can be to tighten the tolerances of the various components to reduce their residual error, such an approach can lead to an indexing system that uses components which are prohibitively expensive and/or require unreasonable manufacturing procedures.
Conventionally, printers have dealt with indexing errors by utilizing multi-pass printing, hereinafter referred to as shingling. For example, in one form of ink jet printer shingling, a fraction of the total number of ink drops are deposited during each of a plurality of passes, while the print receiving medium is indexed by a corresponding fraction of the printhead height. However, as can be understood, shingling can reduce system throughput because of the additional printhead traverses.
Another conventional approach to solving problems associated with indexing errors has been to use a DC servomotor to drive a feed roller, for example, and to mount a rotary encoder disc directly on the shaft of the feed roller. In this approach, an encoder sensor can provide feedback information to a closed-loop control system that can control the angular position of the feed roller for each indexing operation. However, this approach does not appear to compensate for errors caused by components between, for example, the encoder disc and the print receiving medium, such as, for example, runout errors associated with an eccentric mounting of the encoder disc, errors associated with an eccentric mounting and diameter variation of the feed roller, bearing clearances, and media slippage. “Runout” is defined as movement of a cylindrical surface of an object in the radial direction, relative to the surfaces that support the object, during one rotation of the object.
An additional approach is purportedly disclosed in U.S. Pat. No. 5,825,378 (hereinafter referred to as “the '378 patent”), entitled “Calibration of Media Advancement to Avoid Banding in a Swath Printer.” The '378 patent appears to relate to a calibration technique for determining media advance calibration in a swath printer that includes drawing a series of lines on media that correspond to an angle of rotation of the platen, and then using an optical sensor to read the actual positions of the lines in order to transmit a correction signal. However, the system disclosed in the '378 patent appears to require an independent calibration for each type of print receiving media used therewith. As disclosed in U.S. Pat. No. 5,598,201 (hereinafter referred to as “the '201 patent”), entitled Dual-Resolution Encoding System for High Cyclic Accuracy of Print-Medium Advance in an Inkjet Printer,” the system apparently disclosed as the preferred embodiment of the '378 patent also appears to be awkward to use. Therefore, it would be advantageous to have a method and system for accurately indexing print receiving media in a printer that is also relatively simple to use.